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-rw-r--r--kernel/cgroup/pids.c354
1 files changed, 354 insertions, 0 deletions
diff --git a/kernel/cgroup/pids.c b/kernel/cgroup/pids.c
new file mode 100644
index 000000000..511af87f6
--- /dev/null
+++ b/kernel/cgroup/pids.c
@@ -0,0 +1,354 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Process number limiting controller for cgroups.
+ *
+ * Used to allow a cgroup hierarchy to stop any new processes from fork()ing
+ * after a certain limit is reached.
+ *
+ * Since it is trivial to hit the task limit without hitting any kmemcg limits
+ * in place, PIDs are a fundamental resource. As such, PID exhaustion must be
+ * preventable in the scope of a cgroup hierarchy by allowing resource limiting
+ * of the number of tasks in a cgroup.
+ *
+ * In order to use the `pids` controller, set the maximum number of tasks in
+ * pids.max (this is not available in the root cgroup for obvious reasons). The
+ * number of processes currently in the cgroup is given by pids.current.
+ * Organisational operations are not blocked by cgroup policies, so it is
+ * possible to have pids.current > pids.max. However, it is not possible to
+ * violate a cgroup policy through fork(). fork() will return -EAGAIN if forking
+ * would cause a cgroup policy to be violated.
+ *
+ * To set a cgroup to have no limit, set pids.max to "max". This is the default
+ * for all new cgroups (N.B. that PID limits are hierarchical, so the most
+ * stringent limit in the hierarchy is followed).
+ *
+ * pids.current tracks all child cgroup hierarchies, so parent/pids.current is
+ * a superset of parent/child/pids.current.
+ *
+ * Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>
+ */
+
+#include <linux/kernel.h>
+#include <linux/threads.h>
+#include <linux/atomic.h>
+#include <linux/cgroup.h>
+#include <linux/slab.h>
+#include <linux/sched/task.h>
+
+#define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
+#define PIDS_MAX_STR "max"
+
+struct pids_cgroup {
+ struct cgroup_subsys_state css;
+
+ /*
+ * Use 64-bit types so that we can safely represent "max" as
+ * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
+ */
+ atomic64_t counter;
+ atomic64_t limit;
+
+ /* Handle for "pids.events" */
+ struct cgroup_file events_file;
+
+ /* Number of times fork failed because limit was hit. */
+ atomic64_t events_limit;
+};
+
+static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
+{
+ return container_of(css, struct pids_cgroup, css);
+}
+
+static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
+{
+ return css_pids(pids->css.parent);
+}
+
+static struct cgroup_subsys_state *
+pids_css_alloc(struct cgroup_subsys_state *parent)
+{
+ struct pids_cgroup *pids;
+
+ pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
+ if (!pids)
+ return ERR_PTR(-ENOMEM);
+
+ atomic64_set(&pids->counter, 0);
+ atomic64_set(&pids->limit, PIDS_MAX);
+ atomic64_set(&pids->events_limit, 0);
+ return &pids->css;
+}
+
+static void pids_css_free(struct cgroup_subsys_state *css)
+{
+ kfree(css_pids(css));
+}
+
+/**
+ * pids_cancel - uncharge the local pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to cancel
+ *
+ * This function will WARN if the pid count goes under 0, because such a case is
+ * a bug in the pids controller proper.
+ */
+static void pids_cancel(struct pids_cgroup *pids, int num)
+{
+ /*
+ * A negative count (or overflow for that matter) is invalid,
+ * and indicates a bug in the `pids` controller proper.
+ */
+ WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
+}
+
+/**
+ * pids_uncharge - hierarchically uncharge the pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to uncharge
+ */
+static void pids_uncharge(struct pids_cgroup *pids, int num)
+{
+ struct pids_cgroup *p;
+
+ for (p = pids; parent_pids(p); p = parent_pids(p))
+ pids_cancel(p, num);
+}
+
+/**
+ * pids_charge - hierarchically charge the pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to charge
+ *
+ * This function does *not* follow the pid limit set. It cannot fail and the new
+ * pid count may exceed the limit. This is only used for reverting failed
+ * attaches, where there is no other way out than violating the limit.
+ */
+static void pids_charge(struct pids_cgroup *pids, int num)
+{
+ struct pids_cgroup *p;
+
+ for (p = pids; parent_pids(p); p = parent_pids(p))
+ atomic64_add(num, &p->counter);
+}
+
+/**
+ * pids_try_charge - hierarchically try to charge the pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to charge
+ *
+ * This function follows the set limit. It will fail if the charge would cause
+ * the new value to exceed the hierarchical limit. Returns 0 if the charge
+ * succeeded, otherwise -EAGAIN.
+ */
+static int pids_try_charge(struct pids_cgroup *pids, int num)
+{
+ struct pids_cgroup *p, *q;
+
+ for (p = pids; parent_pids(p); p = parent_pids(p)) {
+ int64_t new = atomic64_add_return(num, &p->counter);
+ int64_t limit = atomic64_read(&p->limit);
+
+ /*
+ * Since new is capped to the maximum number of pid_t, if
+ * p->limit is %PIDS_MAX then we know that this test will never
+ * fail.
+ */
+ if (new > limit)
+ goto revert;
+ }
+
+ return 0;
+
+revert:
+ for (q = pids; q != p; q = parent_pids(q))
+ pids_cancel(q, num);
+ pids_cancel(p, num);
+
+ return -EAGAIN;
+}
+
+static int pids_can_attach(struct cgroup_taskset *tset)
+{
+ struct task_struct *task;
+ struct cgroup_subsys_state *dst_css;
+
+ cgroup_taskset_for_each(task, dst_css, tset) {
+ struct pids_cgroup *pids = css_pids(dst_css);
+ struct cgroup_subsys_state *old_css;
+ struct pids_cgroup *old_pids;
+
+ /*
+ * No need to pin @old_css between here and cancel_attach()
+ * because cgroup core protects it from being freed before
+ * the migration completes or fails.
+ */
+ old_css = task_css(task, pids_cgrp_id);
+ old_pids = css_pids(old_css);
+
+ pids_charge(pids, 1);
+ pids_uncharge(old_pids, 1);
+ }
+
+ return 0;
+}
+
+static void pids_cancel_attach(struct cgroup_taskset *tset)
+{
+ struct task_struct *task;
+ struct cgroup_subsys_state *dst_css;
+
+ cgroup_taskset_for_each(task, dst_css, tset) {
+ struct pids_cgroup *pids = css_pids(dst_css);
+ struct cgroup_subsys_state *old_css;
+ struct pids_cgroup *old_pids;
+
+ old_css = task_css(task, pids_cgrp_id);
+ old_pids = css_pids(old_css);
+
+ pids_charge(old_pids, 1);
+ pids_uncharge(pids, 1);
+ }
+}
+
+/*
+ * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
+ * on cgroup_threadgroup_change_begin() held by the copy_process().
+ */
+static int pids_can_fork(struct task_struct *task, struct css_set *cset)
+{
+ struct cgroup_subsys_state *css;
+ struct pids_cgroup *pids;
+ int err;
+
+ if (cset)
+ css = cset->subsys[pids_cgrp_id];
+ else
+ css = task_css_check(current, pids_cgrp_id, true);
+ pids = css_pids(css);
+ err = pids_try_charge(pids, 1);
+ if (err) {
+ /* Only log the first time events_limit is incremented. */
+ if (atomic64_inc_return(&pids->events_limit) == 1) {
+ pr_info("cgroup: fork rejected by pids controller in ");
+ pr_cont_cgroup_path(css->cgroup);
+ pr_cont("\n");
+ }
+ cgroup_file_notify(&pids->events_file);
+ }
+ return err;
+}
+
+static void pids_cancel_fork(struct task_struct *task, struct css_set *cset)
+{
+ struct cgroup_subsys_state *css;
+ struct pids_cgroup *pids;
+
+ if (cset)
+ css = cset->subsys[pids_cgrp_id];
+ else
+ css = task_css_check(current, pids_cgrp_id, true);
+ pids = css_pids(css);
+ pids_uncharge(pids, 1);
+}
+
+static void pids_release(struct task_struct *task)
+{
+ struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));
+
+ pids_uncharge(pids, 1);
+}
+
+static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
+ size_t nbytes, loff_t off)
+{
+ struct cgroup_subsys_state *css = of_css(of);
+ struct pids_cgroup *pids = css_pids(css);
+ int64_t limit;
+ int err;
+
+ buf = strstrip(buf);
+ if (!strcmp(buf, PIDS_MAX_STR)) {
+ limit = PIDS_MAX;
+ goto set_limit;
+ }
+
+ err = kstrtoll(buf, 0, &limit);
+ if (err)
+ return err;
+
+ if (limit < 0 || limit >= PIDS_MAX)
+ return -EINVAL;
+
+set_limit:
+ /*
+ * Limit updates don't need to be mutex'd, since it isn't
+ * critical that any racing fork()s follow the new limit.
+ */
+ atomic64_set(&pids->limit, limit);
+ return nbytes;
+}
+
+static int pids_max_show(struct seq_file *sf, void *v)
+{
+ struct cgroup_subsys_state *css = seq_css(sf);
+ struct pids_cgroup *pids = css_pids(css);
+ int64_t limit = atomic64_read(&pids->limit);
+
+ if (limit >= PIDS_MAX)
+ seq_printf(sf, "%s\n", PIDS_MAX_STR);
+ else
+ seq_printf(sf, "%lld\n", limit);
+
+ return 0;
+}
+
+static s64 pids_current_read(struct cgroup_subsys_state *css,
+ struct cftype *cft)
+{
+ struct pids_cgroup *pids = css_pids(css);
+
+ return atomic64_read(&pids->counter);
+}
+
+static int pids_events_show(struct seq_file *sf, void *v)
+{
+ struct pids_cgroup *pids = css_pids(seq_css(sf));
+
+ seq_printf(sf, "max %lld\n", (s64)atomic64_read(&pids->events_limit));
+ return 0;
+}
+
+static struct cftype pids_files[] = {
+ {
+ .name = "max",
+ .write = pids_max_write,
+ .seq_show = pids_max_show,
+ .flags = CFTYPE_NOT_ON_ROOT,
+ },
+ {
+ .name = "current",
+ .read_s64 = pids_current_read,
+ .flags = CFTYPE_NOT_ON_ROOT,
+ },
+ {
+ .name = "events",
+ .seq_show = pids_events_show,
+ .file_offset = offsetof(struct pids_cgroup, events_file),
+ .flags = CFTYPE_NOT_ON_ROOT,
+ },
+ { } /* terminate */
+};
+
+struct cgroup_subsys pids_cgrp_subsys = {
+ .css_alloc = pids_css_alloc,
+ .css_free = pids_css_free,
+ .can_attach = pids_can_attach,
+ .cancel_attach = pids_cancel_attach,
+ .can_fork = pids_can_fork,
+ .cancel_fork = pids_cancel_fork,
+ .release = pids_release,
+ .legacy_cftypes = pids_files,
+ .dfl_cftypes = pids_files,
+ .threaded = true,
+};